Together with increase of the speed of signal transmission in electronic equipment, differential transmission is generally used as a technology for compensating for degradation of the S/N ratio arising from increase of transmission loss. In the differential transmission, two lines including a P line and an N line are used as one set and transmit signals having phases opposite to each other therethrough. As a result, common noise having a substantially same phase appears on the two lines on the reception side. The common noise is cancelled by taking the difference between the signals. Consequently, the S/N ratio can be improved.
For example, in a wiring board including a glass cloth and differential lines, the dielectric constant of the glass cloth is generally higher than that of resin, and therefore, the propagation delay is small in a region in which the glass cloth is small in amount but is great in a region in which the glass cloth is great in amount. Therefore, a propagation delay time difference (skew) between the lines is generated and the cancellation rate of common noise drops. Together with increase of the speed of signal transmission in recent years, an influence of the skew on a transmission characteristic cannot be ignored.
Therefore, a transmission simulation that takes a skew into consideration is required in order to carry out transmission path design with high accuracy. For example, in order to predict a skew in a wiring board including a glass cloth and differential lines, the internal structure of the wiring board is modeled using a three-dimensional numerical analysis tool to perform electromagnetic analysis.